It says i seize all occupy lands. question is do i have to occupy all baron-tier lands aswell in order to seize whole county or occupiying the capital is enough?
I'm not sure I understand but I think the answer is you need to seize as much land until you have the warscore to force them surrender just like any other war.
Occupy all of them.
My Advice is occupy every single holding, ensuring that any Vassal left will be 100% loyal to you.
EX, If you play as William the Bastard, occupy every English Barony so that you will not deal with the former king/dukes/count revolting, and it will ensure that your vassals will be handpicked by you, which cuts down on revolt risk.